Conventionally, a blade 10 of a bulldozer is removably installed so as to pivot in the forward and backward directions at respective ends of the left and right frames 15A and 15B as shown in FIGS. 6 and 7A to 7C. One end of each of the left and right hydraulic lift cylinders 21A and 21B is removably and pivotably installed at the back of the blade 10 and the other end of each of these cylinders is removably and pivotably installed in a vehicle body 20x. The blade 10 is moved upwardly and downwardly by a contraction/expansion driving of the left and right lift cylinders 21A and 21B.
One end of each of the left and right hydraulic pitch cylinders 20A and 20B is removably and pivotably installed at the back of the blade 10 and the other end of each of these cylinders is removably and pivotably installed in a respective one of the frames 15A and 15B. If the left and right pitch cylinders 20A and 20B are simultaneously extended for driving, the blade 10 is put into a pitch dump position (leaning forwardly) for dumping earth and sand. If the left and right pitch cylinders 20A and 20B are simultaneously retracted for driving, the blade 10 is put into a pitch back position (leaning backwardly) for carrying earth and sand. If the blade 10 has a nose angle .alpha. of 55 degrees, the blade 10 is in an excavation position for excavating earth and sand.
Furthermore, if the right pitch cylinder 20B is stopped without being supplied with any pressurized oil while only the left pitch cylinder 20A is supplied with pressurized oil for extension, the blade 10 makes a rightward tilting action (the right end of the blade 10 tilts downwardly). On the contrary, if the left pitch cylinder 20A is retracted, the blade 10 makes a leftward tilting action (the left end of the blade 10 tilts downwardly).
A request has been made for increasing the amount of carried earth and sand, by enlarging the pitch back angle of the blade 10, since a bulldozer carries the earth and sand for a long distance in a work of topsoiling, such as a strip mining. Therefore, the blade 10 is made with the maximum pitch back angle of approximately 45 degrees, at which the blade 10 is close to the vehicle body 20x.
However, if the blade 10 is operated with a tilting action from a state of the maximum pitch back angle, the blade 10 further approaches the vehicle body 20x as shown in FIG. 8, and then it interferes with a radiator guard or the like in front of the vehicle body. This interference easily occurs since the blade 10 exceeds a tilt angle limit value while it is operated at a higher tilt speed. On the other hand, if the blade 10 is operated at a lower tilt speed, it stops before an operator achieves the desired tilt angle, and therefore the operator cannot achieve the desired tilt angle.
As described above, it is hard even for a skilled operator to achieve the optimum control of the tilt speed and the tilt angle with a speedy tilting action. In other words, due to the great inertia force of the blade 10, an operator cannot stop the supplying of the pressurized oil to the left and right pitch cylinders 20A and 20B with a lever control before the blade 10 interferes with the radiator guard or the like. It occurs due to a requirement of approximately 0.5 second for a response due to properties of oil pressure in conventional hydraulic devices.
To solve these problems, frames 15A and 15B are made longer as shown in FIG. 10B so as to achieve a longer distance between the blade 10 and the vehicle body 20x of the bulldozer 20, whereby the interference can be prevented.
Assuming that W is a weight of the bulldozer 20, Lw is a distance between the center of an actuating wheel 20y and a position of the center of gravity G, and Lf is a distance between the center of the actuating wheel 20y and a nose of the blade 10, the following relation is satisfied: EQU F=(Lw/Lf)W
This relation, however, suggests a problem in that, if the distance Lf between the center of the actuating wheel 20y and the nose of the blade 10 becomes longer, the nose force F of the blade 10 is reduced, which lowers an operation capability. In addition, it makes the length L1 longer for moving the blade 10 rotatably around point pins 15a and 15b of the frames 15A and 15B, which increases the sensitivity of the movement amount of the blade 10 for the control amount of the operator and therefore it becomes harder to control the blade.
Accordingly, to secure the nose force F of the blade 10, the blade 10 must be put into a position close to the vehicle body 20x as shown in FIG. 10A. In the Japanese Non-examined Patent Publication No. 2-204534, this applicant proposed a control device, having functions of tilting and pitching actions of the blade 10, for this type of a bulldozer 20.
To increase the capacity of pressed earth of the blade 10 of the bulldozer 20, it is also required to increase the pitch back angle and the tilt speed. Additionally, in greatly changing the tilt angle of the blade 10, the operator must stop the tilting action so as to prevent the tilt angle of the blade 10 from exceeding the tilt angle limit value. At this point, it is important to determine when the tilting action should be stopped. However, since the tilt angle limit value depends upon the pitch back angle of the blade 10, it is hard for an operator to determine it by himself. To stop the blade 10 so as not to interfere with the radiator guard, the operator must repeat the tilt action many times.
Furthermore, a higher tilt speed of the blade 10 causes the tilt angle to exceed the tilt angle limit value, while a lower tilt speed thereof requires a long time for obtaining a tilt angle desired by the operator, by which it becomes harder to control the blade. This causes a need for expensive devices such as a proportioning control valve or the like.